The present longitudinal study investigated the role of security of child-mother attachment in the development of infant-firstborn relations in two-child families. Its aims are threefold: 1) To assess the extent to which security of firstborn-mother attachment can predict the quality of firstborns' reaction to the birth of the second child; 2) to examine the joint impact of firstborn-mother and infant-mother attachment on the development of infant-sibling relations during the first two years of the infant's life; and 3) to assess the degree to which the quality of early sibling relationships is sensitive to maternal psychosocial functioning (e.g., life stress, marital harmony, psychiatric functioning) and to longitudinal changes therein. Subjects will be 200 two-parent families recruited over a 3-year period when mothers are in their second trimester of pregnancy with their second child. Firstborns will be between 2-4 years of age at recruitment. Each family will be visited at the second and third trimester of pregnancy and at 1-2, 6-8, 12-14, and 21-24 months post-partum. Analyses will focus on predicting the quality of infant-firstborn relations from the quality of each child's attachment to mother, with firstborn perspective-taking ability as a potentially important mediating variable. Structural equation modeling will also be employed to explore directions of influence among measures of maternal functioning, child-mother interaction, child-mother attachment, and infant-sibling relations.